1. Field of the Invention
The present invention relates to communication systems employing power cables for conveyor carriages. More particularly, the present invention pertains to a system for conducting communications between conveyor carriages and fixed stations which superimposes for this system signals on the electric power supplied through the power cables for this system.
2. Description of the Related Art
Conveyor carriages that travel along a suspended rail in a factory or in a storage area to transfer articles between stations are known in the prior art. A fixed station is provided on the ground to communicate with and control the operation of each carriage. In the prior art, communication between the fixed station (key station) and the carriages (local stations) is carried out through a trolley wire that extends along the rail. However, contact between the trolley wire and a collecting brush tends to cause abrasion of the trolley wire. In addition, the installation of the trolley wire and the power cable is burdensome since they must be installed separately along the rail.
Japanese Unexamined Patent Publication Nos. 5-344603 and 6-153305 describe a system that enables the carriages to produce electric power through electromagnetic induction. In the system, each carriage is provided with a pickup coil that is opposed to a power cable in a non-contacting state to induce electric power from the cable.
The present applicant proposed a system for carrying out communication through the power cable without using a trolley wire. In this proposal, a fixed station and the carriages are each provided with an antenna to superimpose communication signals on the high frequency electric power flowing through the power cable. The communication signals exchanged between the carriages and the fixed station are transmitted and received through the antennas.
However, there are certain disadvantages in such a communication system. For example, as shown in the conveying system of FIG. 1, a plurality of power cables 82a, 82b are employed when using a relatively long rail. Electric power sources 81 are connected to cables 82a, 82b, respectively. The cables 82a, 82b are arranged along predetermined sections of a rail 84, along which carriages 83 travel. If only a single cable were to be installed along such a long rail, this would increase the length of power transmission and cause problems such as voltage drop in the cable. To solve such problems, a large electric power source may be employed to increase the amount of electric current supplied to the power cable. However, this would increase the diameter and weight of the cable and add to the burden of the installation of the cable. Thus, the plural cable method shown in FIG. 1 solves voltage drop problems and installation problems.
However, when using the method of FIG. 1, each cable 82a, 82b must be provided with an antenna 85. The cables 82a, 82b are connected to the associated antenna 85, which is located near its power source 81, and to the fixed station 87 by way of a modem 86. Furthermore, when the distance between the antenna 85 and the fixed station 87 is long, a relay is necessary to maintain the strength of the communication signals. In addition, when the rail branches off into a plurality of rails, a power source and a power cable must be provided for each additional rail. This again leads to the problems described above.